1. Field of the Invention
The present invention relates to multilayer ceramic electronic components and mounting structures and methods for the electronic components. In particular, the present invention relates to a multilayer ceramic electronic component, such as a monolithic ceramic capacitor, a structure in which the electronic component is mounted on a substrate by soldering, and a method for mounting the electronic component on the substrate by soldering.
2. Description of the Related Art
A monolithic ceramic capacitor is an example of multilayer ceramic electronic components. Referring to FIG. 5, a monolithic ceramic capacitor 1 includes a base composite of ceramic layers 2 and internal electrodes 3. The internal electrodes 3 are arranged across the thickness of the base composite, are alternately led to either end surface of the base composite, and are connected to external electrodes 4 disposed at both end surfaces of the base composite. The monolithic ceramic capacitor 1 stores electric charge between the external electrodes 4.
The external electrodes 4 of the monolithic ceramic capacitor 1 are connected to electrodes 7 formed on a substrate 6 with solder joints 5. If the surfaces of the base composite and the edges of the external electrodes 4 are covered with an epoxy resin layer 8 (for example, see Japanese Unexamined Patent Application Publication No. 8-162357), the soldered parts of the surfaces of the external electrodes 4 are smaller than an arrangement which does not include the epoxy resin layer 8. Such small soldered parts decrease thermal stress applied to the base composite by heating and cooling and physical stress applied to the base composite by the expansion and contraction of the substrate 6 during the soldering of the monolithic ceramic capacitor 1. This epoxy resin layer 8, therefore, reduces the impairment of the base composite by these stresses.
Referring to FIG. 6, a synthetic resin layer 9 of, for example, polyethylene may be formed on the overall surface of the monolithic ceramic capacitor 1 (for example, see Japanese Unexamined Patent Application Publication No. 9-69468). This synthetic resin layer 9 prevents the oxidation of the external electrodes 4, and thus maintains good wettability of the solder used. In addition, heat that is applied for the soldering melts or decomposes portions of the synthetic resin layer 9 covering the external electrodes 9. Referring then to FIG. 7, consequently, these portions are removed to maintain good solderability.
According to the mounting structure in FIG. 5, however, the external electrodes 4 are exposed to the outside. These external electrodes 4, therefore, are readily oxidized which deteriorates the solderability. In addition, the external electrodes 4, having small soldered parts, increase the possibility of tombstoning of the monolithic ceramic capacitor 1. Tombstoning is a phenomenon where a soldered component is raised by the surface tension of a molten solder. According to the mounting structure in FIG. 7, the edges of the external electrodes 4 are covered by the solder joints 5, but are not covered by the synthetic resin layer 9 after the removal of the heated portions of the synthetic resin layer 9 around the solder joints 5. If, therefore, high voltage is applied on the monolithic ceramic capacitor 1, surface leakage (discharge across the external electrodes 4) readily occurs, which reduces the voltage resistance.
Furthermore, in recent years, efforts to decrease environmental pollutants contained in electrical appliances and electronic components have been promoted to protect the global environment and the human body. One such effort is the use of lead-free solder alloys for mounting. Examples of practical lead-free solder alloys include Sn—Ag alloys, Sn—Ag—Cu alloys, and Sn—Cu alloys for flow soldering, and Sn—Ag alloys, Sn—Ag—Cu alloys, Sn—Ag—Bi alloys, and Sn—Zn alloys for reflow soldering. These solder alloys, however, have inferior wettability than conventional Sn—Pb alloys. Therefore, if the surfaces of the external electrodes 4 are oxidized, the use of these solder alloys may lead to unsuccessful mounting.